Since the 1970's, graphitic nanotubes and fibrils have been identified as materials of interest for a variety of applications. Sub-micron graphite fibrils are sometimes called vapor grown carbon fibers (e.g., nanofibers). Carbon fibrils generally comprise vermiculitic carbon deposits having diameters on the order of about less than 1.0 μm and have typically been prepared through catalytic decomposition of various carbonasceous gases on, for example, metal surfaces. Such vermiculitic carbon deposits have generally been observed since the advent of electron microscopy. See, for example, Baker and Harris, “Chemistry and Physics of Carbon”, 14, 1978; and N. Rodriguez, J. Material Research, 8, 1993.
In 1976, Endo et al. proposed a basic mechanism by which carbon fibrils are thought to grow. See, A. Obelin and M. J. Endo, “Of Crystal Growth”, 32, 1976. Carbon fibrils were generally first observed to originate from metal catalyst particles which in the presence of a hydrocarbon gas became supersaturated with carbon. A cylindrically-ordered graphitic core was extruded and subsequently coated with an outer layer of pyrolytically deposited graphite. These fibrils typically demonstrated diameters on the order of 0.1 μm, and more typically between 0.2 to 0.5 μm.
In 1983, Tennent succeeded in growing cylindrically-ordered graphite cores generally uncontaminated with pyrolytic carbon. See, for example, U.S. Pat. No. 4,663,230. Accordingly, Tennent generally provided access to smaller diameter fibrils, typically on the order of 35 to 700 Å (e.g., 0.0035 to 0.070 μm), as well as an ordered “as-grown” graphitic surface. Fibrillar carbon species of somewhat irregular structure, but without pyrolytic carbon, have also been generally observed.
Carbon fibrils, ‘buckytubes’ (e.g., CNT's) and nanofibers are generally distinct from continuous carbon fibers otherwise commercially available as, for example, reinforcement materials. In contrast to fibrils which usually have large yet generally finite aspect ratios, continuous carbon fibers typically demonstrate aspect ratios on the order of about 104 and often as much as 106 or more. The diameter of continuous carbon fibers is also generally substantially larger than that of fibrils; usually greater than about 1.0 μm and more typically between 5 to 7 μm. Carbon nanotubes of a morphology similar to catalytically grown fibrils have been demonstrated to grow in a relatively high temperature carbon arc. See, for example, lijima, Nature, 354, 56, 1991. It is generally accepted that arc-grown nanofibers have morphology substantially similar to the earlier catalytically grown fibrils originally observed by Tennent. See, for example, Weaver, Science, 265, 1994.
Raw carbon nanotube and carbon nanofiber (CNF) reaction product typically contains numerous reaction byproducts and other contaminants, such as, for example: amorphous carbon; fullerenes; carbon polyhedra; and (in the case of single-wall CNT's) metal catalyst particles. Accordingly, many practical applications require purification in order to effectively reduce these contaminants prior to use of the nanomorphic carbon material. One such method involves a process for purifying carbon nanotubes by generally mixing CNT reaction product with a reagent selected from the group consisting of oxidation agents, nitration agents and sulfonation agents in liquid phase. See, for example, U.S. Pat. No. 5,698,175 to Hiura et al. The CNT's are then reacted at a predetermined temperature in liquid phase, wherein the carbon impurities may generally be selectively dissolved and then subsequently partitioned.
Other conventional chemical purification mechanisms generally involve reaction with an oxidative gas such as oxygen, steam or the like at relatively high temperature. For a general introduction and survey of various CNT and CNF purification methods in terms of their capacity, efficiency and effects on carbon nanomorphs, see for example, G. S. Duesberg et al., “Towards Processing of Carbon Nanotubes for Technical Applications”, Appl. Phys., A, 69, 269, 1999.
Production methods for nanomorphic carbon species are now generally well established and typically allow for synthesis on a relatively large scale on the order of grams per day. For many potential applications of these materials, non-reactive purification still remains a largely unresolved problem. Accordingly, a representative deficiency of the prior art involves the cost-effective and efficient non-oxidative cleaning of, for example, CNT's and CNF's.